A typical automatic transmission includes a hydraulic control system that is employed to provide cooling and lubrication to components within the transmission and to actuate a plurality of torque transmitting devices to allow synchronization for shifting between multiple gear ratios for transmission output. These torque transmitting devices may be, for example, friction clutches and brakes arranged with gear sets or in a torque converter. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. Via multiple valves and multiple solenoid valves individually controlling every clutch control valve, the pressurized hydraulic fluid is directed through a hydraulic fluid circuit to control shifting via torque transmitting devices. The pressurized hydraulic fluid is also directed to various subsystems including lubrication subsystems, cooling subsystems, and torque converter clutch control subsystems. The pressurized hydraulic fluid delivered to the shift actuators is used to engage or disengage the torque transmitting devices in order to obtain different gear ratios.
Known automatic transmissions generally operate in a plurality of modes of operation including out-of-park driving modes and a park mode. The out-of-park driving modes generally include the forward gear or speed ratios (i.e. a Drive mode), at least one reverse gear or speed ratio (i.e. a reverse mode), and a Neutral mode. Selection of the various driving modes is typically accomplished by engaging a shift lever or other driver interface device that is connected by a shifting cable or other mechanical connection to the transmission. Alternatively, the selection of a driving mode may be controlled by an electronic transmission range selection (ETRS) system, also known as a “shift by wire” system. In an ETRS system, selection of the driving modes is accomplished through electronic signals communicated between the driver interface device and the transmission.
While previous transmission control systems are useful for their intended purpose, there is a need for new and improved hybrid transmission hydraulic control system configurations within transmissions which reduce the quantity of control valves and simplify control of the clutch control system, including during transmission default conditions. A default condition is a hydraulic state that the transmission experiences in the absence of electronic control. A transmission in default no longer has the capability to electronically command solenoids to achieve the desired gear state. The default condition may have been intentionally commanded (e.g. when diagnostics indicate corrupt solenoid drivers, corrupt controllers, controller shutdown at high temperatures) or can occur unintentionally due to a hardware failure (e.g. controller failure, wiring harness failure, solenoid driver failure). Accordingly, there is a need for an improved hydraulic control system for use in a hybrid hydraulically actuated automatic transmissions that reduces solenoid valve complexity and can provide different drive states during a default condition.